M.de L. Brooke Brooke, M.de L. Rat eradication in the Pitcairn Islands, South Pacifi c: a 25-year perspective Rat eradication in the Pitcairn Islands, South Pacifi c: a 25-year perspective M.de L. Brooke Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK. <[email protected]>. Abstract This essay off ers a 25-year overview of eff orts to remove Pacifi c rats (Rattus exulans) from the four islands of the Pitcairn group. Following the 1991–1992 discovery that rats were severely reducing breeding success of gadfl y petrels (Pterodroma spp.), Wildlife Management International proposed eradication. Eradication success was achieved using ground-based baiting on the small atolls of Ducie and Oeno in 1997, and there is now evidence of petrel recovery on Oeno, but two eradication attempts on inhabited Pitcairn (1997 and 1998) failed. By the early 2000s, the development of aerial baiting through the 1990s placed an eradication operation on the fourth island, Henderson, within reach. Preparatory fi eldwork in 2009 allayed doubts in two key areas: the feasibility of maintaining a captive “back-stop” Henderson rail (Porzana atra) population, and bait uptake by crabs (Coenobita spp.). Royal Society for the Protection of Birds (RSPB) expertise secured the necessary funding of £1.5 million, and 75 tonnes of brodifacoum-containing bait were dropped in August 2011. Despite extensive mortality of free-living rails, the population, supplemented by released captive birds, returned to pre-operational levels in 2–3 years. Meanwhile those tending captive rails saw no rat sign before leaving Henderson in November 2011. Unfortunately, a rat was sighted in March 2012, and continuing rat presence confi rmed in May 2012. Subsequently rat numbers have returned to pre-operational levels without any sign of population ‘overshoot’ as observed on Pitcairn. Genetic analysis suggests around 80 rats, roughly 1 in 1,000, survived the bait drop. With no evidence of imperfect bait coverage or defi ciencies in bait quality or brodifacoum resistance, it seems some animals chose not to eat bait. Choice tests on Henderson Island rats suggest some rats prefer natural foods over bait. This adverse situation may have been exacerbated because, in August 2011, natural fruits were more abundant than anticipated due to drought earlier in the year. To overcome rat preference for natural food, any second Henderson attempt might benefi t from more attractive bait. Without such developments, a second attempt risks another failure. Henderson’s biota will survive the delay. Keywords: brodifacoum, Ducie, Henderson, Henderson rail, Oeno, Pterodroma INTRODUCTION The Sir Peter Scott Commemorative Expedition to the Observations on the atolls of Oeno and Ducie were too Pitcairn Islands of 1991–1992 involved 35 personnel in intermittent to establish whether rats there had a similar the fi eld over a span of 15 months. While short periods impact on the breeding success of petrels. However, the were spent on the sole inhabited island of Pitcairn (500 ha) fact that petrel densities were 1–2 orders of magnitude and the low atolls of Oeno (c. 60 ha) and Ducie (c. 75 ha), higher on Oeno and Ducie than on Henderson prior to the Henderson Island (4300 ha) was the principal study site. eradications on the atolls suggested that rat impact was Since Henderson had been designated a World Heritage less, if not negligible. Probably because of the presence Site in 1988 “as one of the last near-pristine limestone of rats and feral cats (Felis catus), petrels do not breed on islands of signifi cant size in the world” (<http://whc. Pitcairn. unesco.org/en/list/487>), it had been appreciated that the natural history of the island was incompletely documented. After these fi ndings had entered the public domain The expedition aimed to rectify this omission, bringing via the expedition report (Pitcairn Islands Scientifi c together expertise in archaeology, geology and many Expedition, 1992) and a special volume of the Biological branches of natural history. Journal of the Linnean Society (Benton & Spencer, 1995), the late Brian Bell of Wildlife Management International One of the Expedition’s unexpected fi ndings was the contacted the author to propose rat eradication in the very low breeding success of gadfl y petrels (Pterodroma Pitcairn Islands (Bell & Bell, 1998). At this time, the mid- spp.) on Henderson: ca. 5% among Murphy’s petrels (P. 1990s, an eradication on Henderson was not feasible using ultima), 10% in Kermadec petrels (P. neglecta), and 15– ground-based methods. Therefore, the proposal was for 20% in Herald (P. heraldica) and Henderson petrels (P. eradications on Oeno and Ducie using tested ground-based atrata) (Brooke, 1995). This was especially concerning in methods to benefi t three gadfl y petrel species but, crucially, the case of Henderson Petrels, split from Herald Petrels not the Henderson Petrel which was not confi rmed as a as a result of expedition work (Brooke & Rowe, 1996), nesting species on either atoll. endemic to Henderson and therefore without any source of immigrants to rescue the situation, and potentially on a downward trajectory to extinction within a few centuries ACTIONS (Brooke, et al., 2010a). Oeno and Ducie Field observations showed that the cause of this low The modest extent and fl at accessible topography of the breeding success was predation by Pacifi c rats (Rattus atolls meant that the proposed eradication campaigns were exulans), introduced to the island by Polynesians settlers likely to be successful, given prior achievements elsewhere about 700–800 years ago (Weisler, 1994). Hatching (Towns & Broome, 2003). The eventual source of funding success was apparently not substantially reduced by rats. was the UK’s Department for International Development Rather, the problem arose in the fi rst week after hatching, (DfID) whose interest lay principally in Pitcairn Island especially when the chick moved from under to beside the and its people. For this reason, the programme linked parent. Then the rats approached, pulled the chick away eradications on Oeno and Ducie, off ering clear biodiversity from the nest site, even in the presence of a brooding gains with limited risk of failure, to an eradication parent, and ate it (Brooke, 1995). attempt on Pitcairn where the risks of failure were higher In: C.R. Veitch, M.N. Clout, A.R. Martin, J.C. Russell and C.J. West (eds.) (2019). Island invasives: scaling 95 up to meet the challenge, pp. 95–99. Occasional Paper SSC no. 62. Gland, Switzerland: IUCN. Island invasives: scaling up to meet the challenge. Ch 1B Rodents: Review because of the rugged and heavily vegetated topography impression among the islanders and indeed myself on a and the complications associated with human presence. visit in 2000 that numbers overshot the status quo ante, Nonetheless the project proceeded in late 1997 with to a startling extent. For example, rats were frequently approximately £100,000 of funding for Pitcairn and Oeno encountered in homes, even in cooking ovens left ajar. A from DfID and a further £20,000 for Ducie from the World possible explanation of this ‘overshoot’, that cannot be Wide Fund for Nature (Bell & Bell, 1998). confi rmed by any formal existing trapping or density data, is that, after the reduction in rat numbers due to baiting, Success was duly achieved on Oeno and Ducie by a large amount of food accumulated, for example on or hand-laying of bait (baiting rate unspecifi ed) on a 25 m below Pitcairn’s abundant fruit trees. This surfeit possibly grid (Bell & Bell, 1998). The Oeno eradication has been nourished the extreme increase in rat numbers. followed by growth of the population of the seabird species most easily censused, Murphy’s petrel, at an annual rate of 6% (Brooke, et al., 2017). There are no post-eradication Henderson census data from Ducie. Following the successful eradication of rats from several large New Zealand islands using aerial baiting Pitcairn techniques during the 1990s (Towns & Broome, 2003) and from 113 km2 Campbell Island in 2001 (McClelland Eradication was not achieved on Pitcairn in 1997. & Tyree, 2002), the possibility of an eradication project There, preceding bait laying, the endeavour of cutting a on Henderson Island using aerial baiting moved up the 25 m grid of paths through the dense scrub cloaking the agenda. A feasibility report delivered a favourable verdict, island’s extremely severe terrain taxed the endurance of subject to two caveats (Brooke & Towns, 2008). The the WMIL team, especially since, in the absence of prior fi rst was that, in the areas of high land crab (Coenobita reconnaissance, the severity of the task ahead had not spp.) density behind Henderson’s beaches, it should be been appreciated. Coverage of the cliff s was probably demonstrated that suffi cient bait could be scattered so that, incomplete. A lesson was learnt: future operations of this even after substantial bait removal by crabs, enough bait magnitude must involve prior on-site reconnaissance by remained to permit all rats to consume a fatal quantity. key personnel. The second concerned the endemic fl ightless Henderson The WMIL team departed shortly after the completion rail (Porzana atra). Given the recorded susceptibility of of bait laying (overall baiting rate not specifi ed), entrusting rails to brodifacoum in cereal bait (Eason, et al., 2002) – the task of follow-up monitoring to the Pitcairn Islanders as would be used in a Henderson operation – there was a (Bell & Bell, 1998). Given the many calls on the islanders’ need to demonstrate that Henderson rails could be caught time, and their lack of appropriate expertise, this strategy and then kept healthy in captivity. In the worst-case was probably a mistake. With the benefi t of hindsight, it scenario, the elimination of the wild population during the would have been better if extra costs had been incurred and eradication operation, the captives, once released after the logistical diffi culties overcome to allow some dedicated disappearance of bait, would become the founders of the team members to remain on Pitcairn to detect any residual new wild population.